I suggest to increase step of increment in mouse.move event, or parametrize it in function of current resolution reached ...
i.e.
private void mouseMove(object sender, MouseEventArgs e)
{
static int lastx,lasty;
int delta = 100;
if (abs(lastx-e.X)>delta || abs(lasty-e.y)>delta)>
{
lastx=e.X;
lasty=e.Y;
...
"your zoom box code"
...
}
}